Improve procstat reporting for the linux cdev file descriptors.
If there is a vnode attached to the linux file, use it to fill
kinfo_file. Otherwise, report a new KF_TYPE_DEV file type, without
supplying any type-specific information.
KF_TYPE_DEV is supposed to be used by most devfs-specific file types.
Sponsored by: Mellanox Technologies
MFC after: 1 week